Mesenchymal stem cells (MSCs) are characterized by their ability to produce daughter stem cells and also to differentiate into many distinct cell types including, but not limited to, osteoblasts, stromal cells that support hematopoiesis and osteoclastogenesis, chondrocytes, myocytes, adipocytes of the bone marrow, neuronal cells and β-pancreatic islet cells.(1-3) Thus, MSCs are able to provide the appropriate number of osteoblasts and stromal cells needed for bone development, bone remodeling and hematopoiesis throughout life.
MSCs are extremely rare in the bone marrow and earlier attempts to expand them ex vivo from rodent or human marrow have proven difficult. Moreover, MSCs tend to lose their stem cell properties under traditional cell culture conditions. This situation has impaired the use of MSCs for practical purposes, such as, for example, therapeutic purposes.
The loss of MSC properties in vitro suggests that a critical feature of the marrow environment in vivo, which is responsible for the retention of stem cell properties, is missing in standard culture systems. The present invention illustrates that mesenchymal stem cells require a specialized microenvironment or niche that supports their self-renewal capability, and maintains their multipotentiality while facilitating differentiation in response to appropriate signals.
MSCs reside within the bone marrow, which consists of stromal cells, adipocytes, vascular elements, and sympathetic nerve cells arrayed within a complex extracellular matirx (ECM).(4,5) The bone marrow ECM may comprise molecules selected from the group consisting of collagens I, III, IV, V and VI, fibronectin, and laminin. The bone marrow ECM may also comprise molecules selected from the group consisting of adhesive proteins, large molecular weight proteoglycans like syndecan and perlecan, and members of the small leucine-rich proteoglycan family including biglycan and decorin.(6,7) 
The present inventors demonstrate that culture of marrow-derived MSCs on a cell-free ECM made by marrow-derived stromal cells promotes self-renewal of MSCs and helps maintain the MSCs in an undifferentiated state. The present inventors further demonstrate that following expansion on this ECM, functional MSCs were increased as evidenced by increased formation of bone and hematopoietic marrow tissue following subcutaneous transplantation of in vitro expanded MSCs to immuno-compromised mice.
Stem cells may divide asymmetrically to give a daughter stem cell and a more differentiated progeny, or symmetrically to give two identical daughter stem cells or two more differentiated cells. Regulation of these events allows preservation of stem cells throughout life, and expansion of stem cells as well as production of differentiated progeny when needed for tissue repair.(1) Various embodiments of the present invention illustrate that culture of MSCs on an ECM made by marrow-derived stromal cells promotes symmetric division to produce identical daughter cells whereas plastic favors production of differentiated progeny by symmetric or asymmetric cell division. Moreover, the MSCs expanded on the marrow ECM retain the ability to form a complete bone like structure comprising a calcified matrix made by osteoblasts, hematopoietic marrow containing adipocytes, and stromal cells that support hematopoiesis and osteoclastogenesis. In contrast, growth of MSCs on tissue culture plastic results in eventual loss of self-renewal capacity and multipotentiality, and this is associated with expression of the osteoblast phenotype. Although cells expanded on plastic did form bone in vivo as previously reported,(8) they made less bone and minimal hematopoietic marrow.
Culture of MSCs in the presence of three-dimensional (3D) stromal cell derived ECM allows for attachment, self-renewal, and retention of multipotentiality of MSCs, whereas culture of MSCs under two-dimensional (2D) conditions with or without certain ECM proteins like type I collagen or fibronectin does not.
Loss of stem cell properties, coincident with so-called “spontaneous” differentiation when MSCs are cultured on plastic, may actually represent the response of MSCs to growth factors produced endogenously in these cultures. Indeed, autocrine/paracrine production of BMP2/4 mediates the production of osteoblastic cells when MSCs are cultured on plastic.(9) BMPs bind strongly to collagen as well as small proteoglycans such as biglycan.(10) Embodiments of the present invention demonstrate that the ECM sequestered the BMP2 produced by cultured marrow cells, and this at least partially explains why MSCs retained an undifferentiated phenotype when cultured on a collagenous ECM. Other pro-differentiating proteins may also be sequestered by the ECM.
MSCs lose their multipotentiality when cultured on tissue culture plastic. Previous attempts to overcome this limitation have utilized culture on fibronectin matrices under low oxygen tension (5%)(11,12) to mimic the microenvironment of the bone marrow,(13) or culture at low seeding density in low serum in the presence of growth factors.(14-16) These conditions permitted expansion of murine and human MSCs for as many as 60 population doublings, but the full differentiation potential and cellular composition of these preparations remains unclear.
The present invention illustrates that the marrow ECM forms part of the niche that supports MSCs in the bone marrow, and that the ECM regulates the balance between replication and differentiation in response to appropriate signals. Consequently, the 3D extracellular matrix culture system described herein provides a system for the expansion of functional MSCs for practical applications. This system is invaluable for identification of the contribution of specific ECM components in regulating the behavior of MSCs. Finally, this system is also useful for identifying the effect of aging and/or hormonal changes on the ability of the marrow ECM to maintain MSC function, and thereby contribute to the development of pathologies such as osteoporosis.